Skip to main content

Full text of "Methods of measuring the plasticity of clays."

See other formats




By F. P. Hall. 


Probably the most characteristic property of clay is plasticity. Numerous methods 
have been proposed for measuring this important property. Criticisms concerning 
the more important methods are given in the first part of this paper. The second part 
is devoted to the presentation of experimental data collected with the use of the 
Bingham plastometer, an instrument that has been recently brought forward for 
measuring the plasticity of paints, greases, and other semirigid materials. The plas- 
tometer is a modified capillary-tube viscosimeter. 

Plasticity has been resolved into its two components by Bingham, and the Bingham 
plastometer is supposed to give a measure of these two components designated as yield 
value and mobility. A certain equation has been developed by Bingham giving the 
relation between the two factors of plasticity of a material and the dimensions of the 
capillary of tube used in the plastometer. This equation, together with a modification, 
has been proven by Bingham to hold for paints, but in the case of clay slips with which 
we are dealing the equation does not hold true, as is shown in this article. This is 
due to an end effect which is not taken into account in the equations, and which we 
have not been able to evaluate. Thus we have not been able to express these two 
factors, viz, yield value and mobility in absolute units. But by using the same 
capillary we have been able to obtain some interesting comparisons of several types 
of clays and the effect of addition of certain materials and reagents to clay slips. We 
believe that by using the same capillary we can determine in a comparative manner 
the relative plasticities of clays. 



I. Introduction 346 

1. Need for a practical method 346 

II. Definitions 346 

III. Methods of measuring plasticity 347 

1. Indirect methods 347 

(a) Dye-absorption test 347 

(b) Methods based on bonding power 348 

(c) MacMichael torsional viscosimeter 348 

2 . Direct methods 349 

(a) Method of G. E. Stringer and W. Emery 349 

(6) Atterberg plasticity method 350 

(c) Emley plasticimeter 351 

(d) Bingham plastometer 353; 

Experimental 355 

Preparation of samples 355 

IV. Conclusions 365 


346 Technologic Papers of the Bureau of Standards. [Vol. i 7 



Although plasticity is a property that is essential to the useful- 
ness of clays, it is one for which, after ages of service, there is no 
generally accepted conception which can be crystallized into a 
definition. It is the property which makes clays workable. A 
material like mud, or dough, or putty that can be formed readily 
by simple, moderate pressure, and which will retain whatever 
shape is thus imparted to it, is called plastic. To the potter or 
the brickmaker, plasticity is practically the same as workability. 
It is commonly judged by the feel in the hands, or the way it 
behaves on the potter's wheel. But such methods are not capable 
of giving anything approaching a reliable measure of plasticity, 1 
and it is becoming increasingly important to measure this property 
so that the plasticity of a clay can be expressed as a definite, 
quantitative value. It is obvious that it would be of advantage 
to a producer to be able to catalogue his different grades with a 
definite figure for the plasticity for each, and it would be an 
advantage to the purchaser to have such information in selecting 
clays for trial or for use. The importance of specifications for 
materials is rapidly becoming more apparent, and while it is con- 
ceivable that specifications for clays might be satisfactory without 
containing a direct plasticity requirement, it is reasonably certain 
that such a requirement would be included if a practical method 
for making the necessary measurements was available. Fur- 
thermore, in any scientific study of clays, for the purpose of 
classification or of utilizing the materials more advantageously in 
the ceramic industries, it is evident that a quantitative expression 
for plasticity would be useful. With these considerations in 
view, the Bureau of Standards has undertaken a study of this 
property. The work which is to be reported in this paper deals 
with analysis of the more important of the proposed methods of 
measurement which have gained some recognition, and particu- 
larly with a study of results that have been obtained in a some- 
what extended series of experiments with the method of Bingham 
and Green. 


While it is a simple matter to state, in a general way, what 
plasticity is, a more searching analysis is required to define the 
property in terms that are capable of mathematical expression. 

1 "Mechanism of plasticity," by Bole, J. Am. Cer. Soc., 5, No. 6; 1923. 

Hain Plasticity of Clays. 347 

In other words, it is easy to define plasticity in such a way that 
it is possible to say whether a given material is of low, moderate, 
or high plasticity, but not so easy to define it so that it would be 
possible to determine just how plastic it is. Different investi- 
gators have devised methods of measurement conforming with 
their own conceptions of what it is, or possibly have, in some 
cases, devised a definition conforming to a proposed method of 
measurement. It will be noted in the discussion of the methods 
referred to in this paper that different methods measure different 
factors. It is possible that from a mathematical standpoint 
plasticity is a definite, hard and fast thing for which only one 
correct conception can be formed, and to which only one essen- 
tially correct definition can be applied. However, plasticity as a 
property is essential to materials used/in a number of industries, 
and the term plasticity has a somewhat different significance in 
connection with different industrial operations. Therefore, the 
fact that a method may not measure plasticity according to the 
strictest interpretation of the term does not justify the conclusion 
that the method is of no practical value. That relation of factors 
which it does measure may be what is essential to a particular 
type of operation, and, therefore, it may serve a definite purpose. 
In the discussion of methods, which is given in this paper, it is 
the purpose to state what each one does determine rather than 
to show how nearly each one conforms to a definition which is 
assumed to be correct. 


A considerable number of methods, some direct and others 
indirect, have been proposed for measuring plasticity, the indirect 
methods being based on the assumption that some other related 
but simpler and more easily measurable property would serve as 
an index to plasticity itself. The most noteworthy of the pro- 
posed indirect methods are those based on (1) colloidal content, 
(2) bonding power, and (3) the so-called viscosity of the clay slip. 


Clays exhibit many properties common to substances in the 
colloidal state, such as hydrolysis, absorption, shrinkage, ability to 
change from sol to gel form and vice versa. The colloidal theory 2 

1 "Theories of plasticity," Transactions of Am.Ceram. Soc, 11, p. 536; 12, p. 814; 14, p. 72 ; 16, p. 65; 
17, p. 616. 

348 Technologic Papers of the Bureau of Standards. [Voi.i 7 

is probably the most widely accepted explanation for the cause of 
plasticity, and the analogy between clays and substances in the 
colloidal state has led a number of investigators to look to the 
field of colloidal chemistry for a possible solution of the problem 
of measuring plasticity. Thus Ashley, assuming the plasticity 
of a clay to be inversely proportional to the grain size, reasoned 
that the clay with the highest colloidal content would be the 
most plastic. 

Plasticity is not, however, entirely due to the presence of 
colloidal matter in clays, though the effects of colloids in increasing 
plasticity can not be denied. In determining the colloid content 
Ashley used the dye-absorption test. The absorption of a 
clay is usually determined by noting the loss of color of a dye 
solution, such as malachite green, and comparing it with another 
similar solution to which a standard clay has been added. It is 
known that different colloids have different absorptive powers 
toward a single dye and it is not reasonable to assume that the 
colloidal matter in all clays is the same; hence this method of 
determining the colloidal content is open to a serious objection 
In other words, in order for this method to be correct the colloidal 
matter in different clays would have to have the same absorptive 
power, and this is not the case. 


In the study of clays for a number of industrial purposes, the 
determination of bonding power, which is the ability of a clay to 
impart strength to a dried mixture made up in part of materials 
less plastic than itself, is extremely important. In most cases 
clays of high plasticity also have high bonding power, but, as is 
recognized by ceramists, 3 bonding power is not identical with 
plasticity and is by no means a reliable index to it. It is therefore 
not expedient to discuss the several proposed methods based on 
bonding-power determination. 


This instrument was first brought forward to determine the so- 
called viscosity of clay slips. It is at present used to determine 
the viscosity of liquids. It will be shown later that clay slips 
are not liquids but plastic substances. The torsional viscosimeter 
determines viscosity by means of a torsional balance, usually 
consisting of two concentric cylinders, one of which has an angular 

3 A. V. Bleininger, B. S. Tech. Papers, No. 144, p. 17. J. W. Mellor, Communication No. 53 from Clay 
and Pottery Laboratory, Stoke-on-Trent, p. 1. 

Ham Plasticity of Clays. 349 

motion about their common axis. One of the cylinders is sus- 
pended by a fine wire, thus forming a torsional pendulum. In 
the MacMichael instrument the outer cylinder rotates at constant 
speed. The inner cylinder rotates until the torsional force in the 
suspending wire balances the viscous resistance, and then 
remains in a fixed position so that a reading may be taken. If, 
in the case of a viscous liquid, the speed of rotation of the outer 
cylinder is plotted against the angular deflection of inner cylinder, 
the result will be linear and the straight line will pass through 
the origin. 

If a plastic substance 4 is tested the result will be linear at high 
speeds but the linear portion of the curve extended will not pass 
through the origin but will intersect the deflection axis at a finite 
point above the origin. Clay slips behave like plastic substances 
in this instrument. Tests made with it at the Bureau of Standards 
on a variety of clay slips showed that the results were not repro- 
ducible, due to what may be called a ''puddling effect." Thus, as 
the outer cylinder rotates, that part of the slip immediately sur- 
rounding the inner cylinder changes in consistency as the stirring 
continues. Another difficulty in testing clay slips in this type of 
instrument is that of filling the outer cup to the proper height. 
In dealing with a substance that does not flow freely under its own 
weight it is very difficult to pour the substance into a container so 
that the entire surface will be at the same level. 

This "puddling effect" observed in this type of instrument is 
likely to be present in any instrument designed for this type of 
substance as long as the agitation is local and does not extend to 
the entire mass. In a clay slip the particles are not free to move 
about; hence we have local agitation within the outer cup imme- 
diately surrounding the inner cylinder, and this agitation changes 
the apparent wetness of this part of the slip. This instrument in 
its present form can not be recommended for the testing of clay 
slips. The curves obtained by testing clay slips with it are very 
similar to those obtained with the Bingham instrument, which will 
be described in another portion of this article. 



It is known that plasticity varies with the proportion of water 
mixed with the clay. The possible plasticity is that which can be 
developed under the best known conditions. Messrs. Stringer and 

4 G. S. J. Perrott and Reinhardt Toressen, J. Ind. and Eng. Cheni., p. 324; 1920. 

3 50 Technologic Papers of the Bureau of Standards. [Vol. i 7 

Emery have measured the relations between the proportion of water 
in the clay mixture and the resistance the clay offers to changing 
its shape, and the deformation the clay can suffer without cracking. 
Their method is, briefly : A sphere of clay mixture is made 2 cm 
in diameter. This sphere is placed upon a glass slab and this 
placed under a vertical piston. Weights are added to the piston 
to cause it to descend upon the clay sphere. The descent of the 
piston is stopped when vertical cracks appear on the edge of the 
disc. The distance moved by the descending piston is taken to 
represent the amount of deformation the clay could suffer without 
cracking and the weight required to compress the clay sphere a 
definite distance is taken to represent the resistance the clay 
offers to changing its shape. They found, by this method, that 
as water is progressively added to a clay the plasticity, as deter- 
mined by them, increases to a maximum and then as more water 
is added the plasticity gradually decreases. The chief difficulty 
with this method seems to be in making the clay sphere. Cylin- 
drical test pieces are not as satisfactory as spherical ones. The 
results are expressed in plasticity units which are empirical. It 
is difficult to obtain concordant results and the method can not 
be regarded as satisfactory without modification. It was found 
that the greater the plasticity of the clay the greater the proportion 
of water required to develop the maximum plasticity. A ball 
clay required 30 per cent of water before becoming sticky, a kaolin 
required 20 per cent, while a very short clay required only 10 per 
cent of water to make it sticky. 


This method is based upon the varying physical behavior of 
clays with different water contents. The range of water content 
over the workable stage is determined. The workable stage is 
bounded by two points which are arbitrarily fixed, one point 
being that at which the clay mass will barely flow, and the other 
the point at which the clay mass can no longer be rolled into 
threads. It is Atterberg's contention that the wider this range 
the more plastic is the clay. The results are expressed in a unit 
designated as the "plasticity number," which is empirical. This 
"plasticity number," along with the per cent water of plasticity, 
is supposed to determine the plasticity of the clay. 

'•> B. S. Tech. Papers, No. 46. 

Ham Plasticity of Clays. 351 

It is evident that in a method of this type too much is left to 
the judgment of the person conducting the tests. For com- 
parative tests this method seems to have met with a fair degree of 


This instrument is described in Bureau of Standards Tech- 
nologic Papers No. 169, by Warren E. Emley. F. A. Kirkpatrick 
and W. B. Orange used this instrument for testing clays and 
limes and made a report of the results in an article in vol. 3, 
Journal of the American Ceramic Society. The results are ex- 
pressed in a term called the " plasticity figure," which is empirical, 
depending upon the dimensions of the instrument. Mr. Emley 
states that the consistency has little influence upon the plasticity. 
This is not consistent with the definition of plasticity as com- 
monly given by the ceramist. The ceramist recognizes the fact 
that the plasticity is greatly affected by the water content of the 
clay mixture. There seems to be a difference in the definition 
of plasticity as given by the ceramist and that given by the 
plasterer. Hence the plasticimeter, while apparently giving good 
practical results with plasters, is not measuring the same prop- 
erty that is designated as plasticity by the ceramist. The plas- 
ticimeter measures the property of retaining water and the resist- 
ance to deformation under pressure that a substance possesses. 
It is yet to be proven whether or not these two properties are an 
index of plasticity. 


If clay be suspended in water, the fluidity is lowered rapidly 
and in a perfectly linear manner. As the proportion of clay in- 
creases a concentration is reached at which the fluidity is zero 
as measured in an ordinary viscosimeter. Thus these instruments 
are not available for measuring the viscosity of a suspension above 
that critical concentration where the substance ceases to act as a 
viscous liquid. Prof. E. C. Bingham has developed an instrument 
for the investigation of substances of plastic nature. In using 
this instrument the rate of flow through a capillary tube under a 
definite constant pressure head is determined. The flow at sev- 
eral pressures having been determined, the volume discharged 
per second is plotted against the pressure producing the flow. 
For true viscous liquids the relation is linear and passes through 
the origin; for plastic substances the relation is linear at high 

6 Proceedings, Am. Soc. Test. Mats.. 19 and 20, Pt. II; 1919 and 1930. 
30147°— 23 2 

352 Technologic Papers of the Bureau of Standards. [Voi.i 7 

pressures, but if this linear portion of the curve is extended to 
the pressure axis it will not intersect the axis at zero but at a 
finite point on the pressure axis. Figure i illustrates this differ- 
ence between the two states of substances. The fact that with 
plastic substances the linear portion of the curve does not pass 
through the origin shows that with substances in this state a cer- 
tain definite pressure must be exceeded before the substance will 
flow. 7 This definite initial friction value that has to be exceeded 
distinguishes the plastic state from the viscous state. The ex- 
periments by Bingham support the definition by Maxwell (Theory 
of Heat) that a plastic body is one in which the body is found to 
be permanently altered when the stress exceeds a certain value. 
The following relation between certain terms is given by Bingham 
and Green : 

Solids (including plastic materials): Liquids. 

Rigidity (R) comparable with Viscosity (77) 

Mobility (/*) comparable with Fluidity (0) 

By reference to Figure 1 the terms will be explained. The 
slope of the line (a) is determined by the fluidity of the liquid, 
while the slope of line (b) is determined by the mobility of plastic 
material, and the intercept of the line (6) upon the pressure axis 
is determined by the initial friction value of the plastic material. 
This initial friction value is also designated as yield value. Of 
two clay slips of equal mobility, the one having the higher yield 
value is the more plastic, while of two clay slips of equal yield 
value, the one having the higher mobility is the more plastic. 8 

Plastic clay in the form of a cylinder or cube, for example, will 
maintain its form, provided pressure due to weight does not 
exceed the yield value. Once the yield value is exceeded the 
material will flow. If the material is very mobile it will flow with 
ease ; if not, a greater pressure will be required to keep the material 
in motion. These two factors, mobility and yield value, may be 
said to roughly define the plasticity of a substance. 

The rate of flow of a plastic material through a capillary tube 
is dependent on the two factors of plasticity, namely, mobility and 
yield value — the dimensions of the capillary, and the pressure 

7 According to the more complete theory of Buckingham, there can always be flow, due to slip at the 
lowest pressures. For a discussion of this phase of the subject, see Buckingham, Proc. A. S. T. M., 1921, 
P. "55- 

8 E. C. Bingham, B. S. Sci. Papers No. 278; 1916. 


Plasticity of Clays. 


applied. Bingham has derived an equation connecting these sev- 
eral factors and based on the following assumptions : (i) That the 
rate of shear in a given plastic material is proportional to the 

FlG. i. — Diagrammatic illustration of the difference between plastic and viscous flow. 
Ordinates are flow values, abscissas are pressures. 

excess of shearing stress over a constant yield value, below which 
the material behaves as a solid, (2) no end effect at the entrance 
of the tube, and (3) no slip or seepage. 


Technologic Papers of the Bureau of Standards. [Vol. ij 

The equation of the straight line, on the assumption that the 
phenomena are not complicated by slip, seepage, or end effects, is: 9 



Where — = flow in cc per second. 

fx = mobility. 

/ = yield value, dynes/cm. 2 

5 = shearing stress at the wall of capillary in dynes/cm. 2 
R = radius of capillary, cm. 

But as the shearing stress S has the following relation to the 
pressure P: 


S = 


Where P = pressure in grams/cm. 2 
L = length of capillary, cm. 

Then by rearranging the terms in equation (i) we obtain the fol- 
lowing equation: 



/» = 


Let Y = 

rR 3 t 

*~if — 

5 PR 

' IT 

Since the mobility is a constant of the material and by equation 

Y Y 
(2) , \x = then the ratio is constant for any given plastic 

3 ; 

3 7 

8 For the derivation of this equation from Bingham's hypothesis, see Buckingham, Proc. A. S. T. M., 1921, 
p. 1155; or Bingham, Fluidity and Plasticity, 1922, pp. 223-225. 

Hdin Plasticity of Clays. 355 

substance and Y is a straight-line function of 5. If Bingham's 
assumptions are correct, then by plotting the factor Y against 5 
for a particular substance, using several different sized capillaries, 
all of the resulting straight lines should coincide and the two 
constants, mobility and yield value, will be independent of the 
dimensions of the capillary. 10 

This theory is correct for paints and for some very fine-grained 
fat clays, but does not hold true for coarse-grained lean clays. 
Experimental data showing this to be true for several clays are 
presented in Figures 2, 3, 4, and 5. 

Experimental. — In order to determine whether or not this 
theory applies to observations on clay slips, it was necessary to 
use several different capillaries with the same slip and then com- 
pare the results obtained with the different capillaries. As water 
content is a very important factor in determining the consistency 
of a clay slip, it is necessary to give this factor with every determi- 

nation. The factor — is used for this purpose and denotes the 

ratio of water to clay by weight. Time is also a factor in determin- 
ing the consistency of a clay slip and this was kept constant by 
carrying out the plastometer tests three days after the slip was 

Preparation of Samples. — In making up the slip the water 
and clay were weighed and mixed and the slip pressed through 
a 150-mesh screen in order to remove the grit that might clog up 
the capillaries. The slip was put into an air-tight container and 
allowed to stand for three days. The consistency of a clay slip 
changes during the first two days, probably due to the swelling 
of the gel colloids. The consistency changes more in the first day 
than later periods. The sample was again stirred and part of it 
was placed in the container of the plastometer for testing. Volume 
discharge through one capillary at several pressures was deter- 
mined and these results checked and then tabulated. The capil- 
lary was changed and this procedure was followed using several 
other capillaries. All of the determinations on one slip must be 
made in one day because of the effect of time on the consistency 
of the slip. 

10 This method of analysis of the plastometer results was suggested by Dr. E. Buckingham, of the Bureau 
of Standards. 

356 Technologic Papers of the Bureau of Standards. [Voi.z 7 

By reference to Figure 2 we see that the results on the same slip 
using different capillaries are not the same and the straight por- 
tions of the curves do not coincide. Only a few of the results of 
tests on the English china clay are presented here. Tests on three 
slips of the English china clay are shown, using two capillaries in 
one case and three in the other two cases. Thus we see that in 
the case of the English china clay there is some effect or effects 
coming in which were not taken into consideration by Bingham 
in the development of equation (1). 

By reference to Figure 3 we see that in the case of the English 
ball clay, that with a high water content the deviation is not great 
and is within the limit of experimental errors. As the water 
content decreases the results deviate considerably, as is shown by 
the two curves on the right of the figure. This deviation is too 
great to assume that it is due to experimental errors. There are 
other effects coming in that were neglected in the theory. Figure 
4 shows the results of tests carried out on D. B. Georgia kaolin. 
We see that in this case we also have a deviation of facts from 
theory. In Bingham's original assumptions, on which he based 
his theory of plastic flow through capillary tubes, no account was 
taken of an end effect. This end effect we believe is the cause of 
the deviations between facts and theory. Small end effects are 
noticeable in the case of viscous substances when they are tested 
in a viscometer with short capillaries and are caused by shearing 
of the liquid before entrance into tube. Similar but much larger 
end effects occur with clay slips, especially with the lean clays. 
In the case of the clay slips it becomes greater as the water con- 
tent is decreased. It is less in the case of the more plastic clays, 
and this may be due to the fact that these clays have a higher 
colloidal content and this colloidal material acts as a lubricant 
between the larger particles. 

By assuming that this end effect is equal to a resistance which 
would be produced if the capillary tube were made longer, we are 
able to reduce these deviations in some cases. One example of 
this is shown in Figure 5. The same values uncorrected are 

shown in Figure 4. The ratio p is increased by adding a constant 
to the ratio -=, for both tubes, and the curves are made to coincide. 

By reference to Figure 5 we see that this constant increases as the 
water content decreases. In other words, the end effect is greater 


Plasticity of Clays. 


d 12 T6 ZO 2* 2d 32 

Fig . 2. — Tests carried out on English china clay with different capillaries at three concentrations. 


Technologic Papers of the Bureau of Standards. ivoi. r? 

i i 

w i $ i s 


Plasticity of Clays. 



Technologic Papers of the Bureau of Standards. [Vol. i 7 

with lower water content. We have not been able to reduce all 
of these results by this assumption. The values shown in Figure 
2 can not be reduced to identical values by this assumption. 
There seem to be other effects coming in which are not accounted 
for by Bingham's original assumption. 

It is Bingham's contention that the slope of the straight line 
portion of the pressure-flow curve is determined by the mobility 
of the material tested and that the intercept of this line on the pres- 
sure axis is determined by the yield value of the material. We 
believe that in the case of these clays an end effect affects both the 

yoc«y 2Z'C 



"zSJf Tube -R- enoUorr fctc 

[ q 1.1 128.2 +Z36.9 2< 
\5IS{ q 4 8 4.9 +/36.S 22 









m f q M 128.2 +J6S.9 3I+.I 
'• 220 \q * 84.9 +1859 270.8 




























10-2 S (corrected) <$£i 



i : 


\ i 


i -j 

r i 

\ s 



( i 

2 1 

■b \ 

+ 15 

Fig. 5. — Effect of increasing the ratio -^ The uncorrected values are shown in Fig. 4. 


slope and intercept of the plastometer curves. Therefore, the slope 
and intercept of the pressure-flow curve will not give us the values 
of the mobility and yield value of a plastic material unless the end 
effect and slip can be evaluated or eliminated. Thus far we have 
not been able to do either. In fact, it is not reasonable to assume 
that all plastic substances will act alike when tested in the plasto- 
meter, and the opposite assumption is supported by the fact that the 
results of ly. Livshis 11 show that cooking fats do not act as paints when 
tested in the plastometer and that the results of Porst and Mosko- 
witz l2 with corn-product starches are not entirely consistent with 
Bingham's equations. In the case of cooking fats there seems to be 

11 Unpublished manuscript of I^.Livshis 12 J. Ind. and Eng. Chem. 14, No. 1. 

Haii] Plasticity of Clays. 361 

an end effect that affects the slope and intercept of the pressure-flow 
curves. Bingham's 13 observations on clay slips can not be reduced 
to absolute values by using the equations which he later derived 
for paints. 

While we have not been able to reduce the observations on clay 
slips to absolute values or values independent of the capillary, we 
have obtained some interesting information by using one capillary 
with several slips and then comparing the results. Figures 6, 7, 
8, and 9 show these results. 

We will not consider yield value and mobility, for we have not 
been able to determine these values for clay slips. We will con- 
sider the slope and intercept of the pressure-flow curves as obtained 
by the use of the Bingham plastometer. Given some particular 
clay and capillary we can vary the slope and intercept of the 
pressure-flow curve by varying the water content of the clay slip 
that is to be tested, but for a given clay we can not alter the func- 
tional relation between the slope and intercept. This was illus- 
trated in Figures 2,3, and 4. Figure 6 shows the result of vary- 
ing the water content of several clay slips until the intercept of 
pressure-flow curves is the same, using the same capillary in every 
case. The slopes of the straight line portions of the pressure-flow 
curves vary over a wide range. It is our belief that, the inter- 
cept being the same, the clay giving the steepest slope will be 
the more plastic. According to this idea of plasticity the relative 
plasticities of the several clays shown in Figure 6 are as follows: 

English ball clay (most plastic) . 

Tennessee ball clay. 

Kentucky ball clay. 

Florida plastic kaolin. 

South Carolina kaolin No. 1. 

South Carolina kaolin No. 2 (least plastic of series). 

Figure 7 shows the effect of varying the water content until the 
slopes of the pressure-flow curves are the same, using the same 
capillary in every case. The order of relative plasticity in this 
case is : 

English ball clay. 

Tennessee ball clay. 

Kentucky ball clay. 

Figure 8 shows the results of observations on mixtures of clay 
and bentonite, and clay and flint. Bentonite is a hydrous alumi- 

13 E. C. Bingham, B. S., Sci. Papers, No. 278; 1916. 


Technologic Papers of the Bureau of Standards. [Voi.i 7 





I I 


ojc/dd /ioy 


Plasticity of Clays. 


num silicate in a very finely divided state. When bentonite is 
added to English china clay the value of the intercept is increased 
and the slope of the pressure-flow curve is changed very little. 



Fig. 7. — Comparison of different clay slips using the same capillary. 
The concentration has been varied until the pressure-flow lines have approximately the same slope. 

In other words, the bentonite increases the plasticity of the china 
clay. The addition of bentonite to a lean clay increases the 
colloidal content of the mixture and at the same time increases the 


Technologic Papers of the Bureau of Standards. [Vol. i 7 

plasticity of the material. The addition of flint has the opposite 
effect. This is shown in Figure 8, where an addition of 10 and 15 
per cent flint decreases the value of the intercept. 





200 400 

600 300 WOO /200 


Fig. 8. — Effect of adding plastic and nonplastic material to an English china clay slip. 

Figure 9 shows the result of adding certain reagents to clay 
slips. Acids are known to flocculate clays while alkalies defloc- 
culate clays. Alum acts like an acid in this respect and we see 
that the addition of alum to a slip increases the intercept, or, in 
other words, increases the plasticity, since the slope is about the 
same as without the alum ; alkalies have the opposite effect, as is 


Plasticity of Clays. 


shown by the figure. The addition of certain organic colloids, 
such as hot starch, gum, tannin, and gelatin to a clay slip reduces 
the value of the intercept and causes the slope of the pressure 
flow curve to be steeper. This action is shown in Figure 9. 




600 /OOO /ZOO 


Fig. 9. — Effect of adding flocculating and deflocculating agents to an English china clay 



It is evident in surveying the methods brought forward for 
measuring this fundamental property — plasticity — that none of 
the methods is entirely satisfactory. It seems that plasticity 
is a resultant of two factors or perhaps more. 


Technologic Papers of the Bureau of Standards. [vol. i 7 

If it be true that plasticity is determined by two or more elements 
or components, then in comparing the relative plasticity of several 
clays it is necessary to have all but one of the components con- 
stant and the plasticity will vary in some manner with this one 
component. This was illustrated when dealing with the Bingham 

With most of the methods it is very difficult to obtain concord- 
ant results. On account of the heterogeneity of the system clay- 
water it is very difficult to formulate mathematical equations that 
will satisfy the observations. 

The Bingham plastometer enables us to detect slight changes 
of consistency in clay slips. We believe we are able to determine 
roughly the relative plasticities of clays when comparing plas- 
tometer curves over the same range of flow and with the same 
capillary, but the results are not independent of the dimensions 
of the capillary used. Thus the results are empirical and are 
comparable only when they are obtained with the same capillary. 

It is highly probable that with the use of some other type of 
plastometer (other than capillary-tube type) that absolute values 
of the two plasticity components could be determined but with the 
capillary-tube type of plastometer the case is complicated by the 
presence of such phenomena as slippage, end effects, etc. 

The writer wishes to express his indebtedness for advice in this 
work to Dr. E. Buckingham, of the Bureau of Standards. 

TABLE 1. — Dimensions of Capillaries Used. 


radius > in 

at 22° C. 

Length in 


at 22° C. 


radius » in 

at 22° C. 

Length in 


at 22° C. 


0. 03794 
. 12897 
. 05492 
. 05857 
. 06118 



0. 04547 
. 12754 
. 10662 
. 05874 
. 07563 




16. 705 









11. 620 


Calculated from major and minor axes of tube; cf. Zeitschr. f. physik. chem., 80, p. 683; 

Washington, November 4, 1922.